1. Field of the Invention
The present invention relates to methods of acquiring subsurface reflection signals in a marine environment and, more particularly, to such methods that utilize towed horizontal receiver arrays in conjunction with vertically disposed receiver arrays to obtain the reflection signals beneath a fixed obstruction.
2. Setting of the Invention
As shown in FIG. 1, a seismic vessel 10 is illustrated towing very long (e.g., about 3 kilometers) cable(s) of seismic receivers 12 along a desired shot line. The receiver array 12 can become entangled in fixed obstructions, such as production platforms, structures and equipment 14 that are used to produce oil/gas. To avoid such entanglement, the vessel 10 must steer away from the structures 14, which thus moves the shot line and the resulting data away from the structures 14, which in turn causes a gap in the seismic data directly beneath the structure 14. Often times one desires to record seismic reflections in and around a known production zone, e.g., directly under the offshore structure 14, but one is prevented because of the entanglement problem.
Further, in the acquisition of subsurface reflection signals in a marine environment, it is well known to utilize linear arrays of horizontal seismic energy receivers, i.e., a plurality of towed streamers wherein each streamer contains a plurality of hydrophones. While these linear arrays of seismic energy receiver elements are especially good at attenuating, i.e., reducing or eliminating, seismic noise waves that travel parallel to or nearly parallel to the linear array, these linear arrays are relatively ineffective at attenuating horizontally traveling seismic noise waves that travel in a nonparallel direction to the linear array.
There is a need for a marine data acquisition method that can be used in close proximity to fixed structures and to attenuate undesired noise.
Vertical arrays of seismic energy receiver elements have been used in the acquisition of subsurface reflection signals in a marine environment. Examples of vertical arrays are disclosed in U.S. Pat. No. 4,694,435; "Research Note: Horizontal and Vertical Arrays at Sea," Merzer, Mar. 18, 1971, Geophysics, J. R. Astrological Society (1972), 29, 367-370; "Seismic Reflection Profiling With a Deep-Towed Vertical Hydrophone Array," Collins et al., Oceanographic Institution, November 1983, Marine Geophysical Research 6 (1984), 415-431; and "A Simple Deep-Towed Vertical Array for Higher-Resolution Reflection Seismic Profiling," Herber et al., Marine Geophysical Research 8 (1986), 175-186.